1. Open the Settings app on your device2. Tap Location3. Uncheck and re-check the on/off option making sure to leave it 'on'4. Open Grindr

Be sure to also check your data connection. When you're using a Wi-Fi network, location is determined by the network's IP address. Disable the Wi-Fi settings on your device and try using Grindr with only a mobile (3G, EDGE) network connection.